Using a linked camera from another file
I got this to work if......

I created a group out of only the camera in the linked file ("A").

In my new file ("B"), I linked in the group from ("A").

In the outliner, I expanded (drilled down into) the linked group. 

Then on the child group object:


Right Clicked.
Select >> Link Group Object To Scene.
Selected the camera object in the 3D view.
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